The present invention relates to a process for production of acrylic acid, a process for production of polymers by polymerization of acrylic acid, preferably for production of water-absorbing polymers, the water-absorbing polymers obtainable by this process, water-absorbing polymers which are based to at least 25 wt. % on partially neutralized acrylic acid, to a composite, a process for production of a composite, the composite obtainable by this process, the use of acrylic acid for production of water-absorbing polymer structures, a device for production of acrylic acid, a process for production of acrylic acid and the acrylic acid obtainable by this process.
High requirements are made of the purity of acrylic acid which is used for production of polymeric compounds. This is particularly the case if the polymers are so-called superabsorbers, which are incorporated in wound dressings or hygiene articles. These polymers are capable of absorbing and thereby binding aqueous liquids to form a hydrogel. Superabsorbers, are, therefore, particularly used in hygiene articles such as diapers, incontinence inserts, feminine hygiene articles and the like for absorption of body fluids. An encompassing over-view of superabsorbers, the application and the production is given by F. L. Buchholz and A. T. Graham (editor) in “Modern Superabsorbent Polymer Technology”, Wiley-VCH, New York, 1998.
In the production of the superabsorbing polymers, generally an acrylic acid is used which has been obtained by catalytic gas phase oxidation of propylene to acrolein, which is then converted in a further catalytic gas phase oxidation to acrylic acid, subsequent absorption of the gaseous reaction mixture in water, distillation of the thus-obtained aqueous acrylic acid solution to obtain a pure acrylic acid and further purification of the crude acrylic acid by distillation or crystallization.
A disadvantage of this process for production of acrylic acid is that the reagent used (propylene) is produced from crude oil and thus from non-sustainable raw materials, which is of concern for economic aspects, above all long-term, above all with respect to the obtainment from crude oil which is becoming increasingly difficult and above all expensive.
Known superabsorbers have the disadvantage that, unless they at least partially comprise natural polymers, such as, for example, celluloses, they are hardly based on renewable raw materials. It is possible to produce many of the components used in hygiene articles, in particular in disposable diapers, from biological starting materials, but a replacement of superabsorbers based on cross-linked polyacrylates by natural superabsorbing polymers, such as, for example, by cross-linked, derivatised starches or celluloses, is generally linked with significant losses with respect to the absorbent properties. This mostly leads to considerably more of these absorbers based on natural polymers having to be used, in order to even approach the same absorbent properties in a hygiene article. This is disadvantageous, because the hygiene articles then become more voluminous and heavier, which significantly reduces wearing comfort and leads to a larger waste volume, which, in addition to more disposal space or combustion expenditure, also makes necessary increased transport capacity for the waste removal. All of this has a disadvantageous effect on the environmental tolerance of absorbers based on natural polymers.